1. Field of the Invention
This invention relates generally to a method and apparatus for reducing noise in a low current circuit, and more particularly, for reducing noise in a low current circuit included within a high current, high frequency integrated circuit chip.
2. Discussion of the Related Art
Oscillators for use in semiconductor clock chips operate at very low current levels, in the range of hundreds of nanoamps. An example of such a semiconductor clock chip includes, for example, a real time clock chip such as part number M48T86, commercially available from SGS-Thomson Microelectronics, Inc. of Carrollton, Tex. To obtain an optimal performance at the very low current levels in the range of hundreds of nanoamps, noise considerations become a serious concern. To address noise considerations, all of the oscillator components, with the exception of the oscillator crystal, are typically located "on-chip" in as low a noise environment as possible. Oscillator circuit supply voltages, Vss and Vcc, are provided to the oscillator circuit components using dedicated voltage supply lines. Furthermore, shielding is an important component of a design of a system clock chip having a low current oscillator circuit. Such a clock chip is also generally implemented in a low noise environment.
Computers and computer workstations typically utilize time keeper parts, i.e., clock chips. With respect to time keeper parts, the parts have a timing function which typically operates with the use of a crystal oscillator and further includes a battery back-up for when a main power is removed (i.e., turned off). Battery back-up enables the clock chip to continue to function even when the main power is off. Typically, the registers that record minutes, hours, days of the week, or days of the month, etc., remain active with the battery back-up so that when the system power comes back up, everything has been updated and is current. In other words, when power comes back up, the time keeper part or clock chip can read out the correct time and date, etc. A longevity of a particular battery recharge and/or battery life is dependent in part upon an amount of current required for operating the clock circuit when a main power is off.
With respect to computers and/or workstations, and in an effort to integrate as many functions as possible in as few chips as possible, it would be desirable to include a low current oscillator in a chip set for a particular microprocessor. The low current oscillator would enable an extended battery life when a main power is down. However, the chip set presents an extremely noisy environment for a low current oscillator to operate within. That is, the chip set includes a high current, high frequency noise environment. The low current oscillator is a fairly slow part, wherein the low current oscillator for a clock chip typically operates in the kilohertz range. A microprocessor and its corresponding chip set on the other hand, typically operate in the megahertz range.
In some instances, certain components of a particular circuit may not be capable of being manufactured on a single integrated circuit chip with a remainder of the integrated circuit components of the particular circuit due to semiconductor manufacturing process limitations. For example, a particular semiconductor manufacturing process may not allow for the formation of high precision capacitors of an oscillator circuit to be formed on the same chip as the main portion of the oscillator circuit, wherein the particular semiconductor manufacturing processor is principally for the making of microprocessors and/or microprocessor chip sets using a particular technology (e.g., 0.5 .mu.m, 0.35 .mu.m, 0.25 .mu.m or less). As a result, the fabrication of the oscillator circuit must be split up, for example, wherein the non-process conforming capacitors are required to be fabricated off-chip. This is a disadvantage, since it would be desirable to make the oscillator circuitry all on a single chip where a very tight control can be exercised over the process used in making the oscillator circuit portion of the integrated circuit chip.
As indicated above, an oscillator circuit may be desirable for inclusion into an integrated circuit chip of a computer chip set, wherein the environment of the integrated circuit chip of the computer chip set is extremely noisy. In the instance of a low current oscillator circuit, which includes two capacitors connected to each side of an oscillator crystal to a power supply ground Vss, the capacitors may require a precision that is not available in some semiconductor manufacturing processes. For example, the capacitors may be fairly large, having a size dimension on the order of 100 .mu.m per side. Such large capacitors require extra processing steps which may not be easily incorporated into or desirable to be included into the same integrated circuit manufacturing process as the integrated circuit chip of a computer chip set. In addition, incorporating the capacitors into the same manufacturing process may affect yield and would also increase the manufacturing costs of the integrated circuit because of the extra processing steps required. Specific requirements may also require that the capacitors from both crystal nodes to Vss be located outside of the chip (i.e., off-chip). In such an extremely noisy environment, the Vss located outside of the chip will have a different absolute value when compared to the same signal inside the chip. Such a difference adversely affects the timing operability of the low current oscillator circuit.
Turning now to FIG. 1, a circuit board 10 is shown having a high current, high frequency integrated circuit chip package 12 mounted thereon. Circuit board 10 contains numerous conductors for interconnection of various component parts. The conductors include, for example, a board level power supply positive potential Vcc line 14 and a board level power supply ground Vss line 16. Package 12 is connected to circuit board 10 via standard techniques.
Referring still to FIG. 1, package 12 includes a high current, high frequency integrated chip 18 mounted thereon. Chip 18 is interconnected with package 12 using bond wires 20, for example, or any other suitable interconnection known in the art. Chip 18 further includes a slow speed, low current oscillator circuit portion 22 integrated therewith. Low current oscillator circuit 22 includes a crystal 24 and capacitors 26, 28 mounted external to chip 18 and package 12 (i.e., off-chip). Capacitors 26, 28 are mounted on circuit board 10, each respectively connected between an opposite end of crystal 24 and board supply ground Vss 16.
Integrated circuit chips are typically bonded to a package and the package is bonded to a circuit board. Any number of integrated circuit chip connections may be employed. Electrical interconnection between the package and the semiconductor chip can be made using wirebonding, flip-chip, thermally activated bonding, solder bumps, and/or other chip-to-package interconnect techniques known in the art. The package includes any suitable input/output connection for getting signals into or out of an attached integrated circuit chip. The package is furthermore interconnected with the circuit board using any suitable techniques known in the art.
A typical frequency of operation of the low current oscillator circuit as discussed herein on the order of tens of kilohertz and more particularly, for example, 32,768 Hz. Such a low current oscillator circuit uses a very low frequency crystal. In a situation wherein the low current oscillator circuit is not able to be entirely fabricated within a desired integrated circuit chip because of semiconductor manufacturing process limitations, as discussed, the environment for which the low current oscillator circuit is intended may include a high current, high frequency environment. Such a high current, high frequency environment typically operates in the megahertz range (e.g., in the tens of megahertz range such as 33, 66, and 100 MHZ, etc.) and includes, for example, a computer or microprocessor chip set. Noise within the integrated circuit chip of a computer chip set will thus be in the very high frequency range. From the above, note that there are quite a few orders of magnitude difference in the operating range of the low current oscillator circuit and the frequency of the noise. The output signal 32 of a low current oscillator circuit without noise is generally in the form of a sine wave, as shown in FIG. 4. Without noise, a trip point 80 is easily recognized, wherein the trip point 80 is determined by the intersection of the oscillator output signal with a prescribed voltage trip level 82. The output signal 34 of a low current oscillator circuit with high frequency noise induced in the output signal is shown, for example, as illustrated in FIG. 5. Note that in FIG. 5, a trip point 84 is not easily recognized in view of the high frequency noise on the oscillator output signal. In reality, the high frequency noise is much greater than that shown.
In general, most of the function of a clock chip is not sensitive to noise, except with respect to the obtaining of an oscillator signal. Once obtained, the oscillator signal is typically divided down to where it starts to pick up amplitude and swings rail-to-rail (i.e., from Vss to Vcc), as is known in the art. The most sensitive portions of a clock chip are the oscillator and the first couple of divider circuits. If noise is introduced into an input of a divider stage, the noise is liable to trip (i.e., trigger) that particular stage multiple times, when the input should only trip that particular stage once as the oscillator circuit output signal passes through the transition or trip point. Power supply connections to a particular high current, high frequency integrated circuit chip are typically achieved using many external power supply pin inputs (not shown). The external pin inputs correspond, for example, to Vss (supply ground) and Vcc (supply voltage). The many external pin inputs are connected to corresponding internal busses on-chip to reduce the amount of power supply noise to an amount on the order of volts, more particularly, on the order of hundreds of millivolts.
In the situation where a power supply is provided to the high current, high frequency chip 18, the power supply can have an absolutely clean output signal, i.e., the output has no appreciable noise on it. By the time the power goes through a combination of the package pins, bond wires, and onto the semiconductor chip die, the clean power supply output signal would no longer be clean. Instead, the power supply signal would have an appreciable amount of noise on it due, for example, to an RCL characteristic of conductive leads and a switching which occurs on the chip. The switching is a function of an output switching of logic circuitry on the chip. On high speed chips, as many as 50-75% (fifty to seventy-five percent) of the outputs typically may be switching at any one time. A chip set for a microprocessor includes such high speed chips, wherein the chip set is used to establish an the environment for the microprocessor (or processor) to function, in accordance with a particular computer and/or computer architecture.
Numerous sources of noise generation exist on-chip. As mentioned above, even if the power supply to the chip was clean (i.e., contained no noise), by the time the power gets onto the chip, there would be noise in the power supply signal. This results in part from inductances on chip connection leads, resistances in conductors and connections, in addition to switching which occurs on-chip, leading to a very noisy environment. FIG. 2 is illustrative of the RCL circuit characteristics of resistance, capacitance, and inductance of the conductive paths. FIG. 3 further illustrates oscillator circuit 22 integrated with chip 18, circuit 22 further including crystal 24 and capacitors 26, 28 connected external to integrated circuit chip 18. Capacitors 26, 28 are connected between opposite ends of crystal 24, respectively, and board Vss supply 16. The current i.sub.IN off-chip will be different from on-chip current i.sub.(chip+OSC) as a result of circuit induced noise from the high current, high frequency, integrated circuit chip 18. FIG. 3 further illustrates the RCL characteristic 30 of the conductive interconnections.
With respect to typical on-chip busses of an integrated circuit chip, noise is dependent upon the specifics of the particular chip. The particular noise can be as much as a volt. With well designed integrated circuit chips, using numerous power supply pins, noise may be maintained to within a level on the order of several tenths of volts. Noise is thus dependent upon numerous factors, including, a particular function of the chip, the particular characteristics of the chip, and upon particular portions of the circuits on the chip which may happen to be operating at any one time, e.g., whether or not there is high speed switching occurring. Functionality of the circuitry on a chip also adds to the noise on the chip, e.g., high current, high frequency circuits, etc.
Referring again to FIG. 1, the slow speed, low current oscillator 22 is integrated on the high current, high frequency chip 18. A noise generated on-chip during operation of the chip 18 could upset the oscillator 22 and a corresponding clock function. In other words, noise problems would occur. While the oscillator circuitry 22 would occupy a small portion of the silicon of the very high speed chip 18, the oscillator circuitry 22 would reside in an environment which has very high switching speeds (i.e., in a range of less than 15 nsec (nanoseconds)). The environment is thus very noisy in terms of the low current, low noise oscillator 22. An amount of excursion at the output of the oscillator 22 is very small, on the order of 0.5 to 1 volt peak to peak with a noise margin on the order of millivolts. A typical output of an oscillator circuit is a sine wave, wherein the excursion amount is plus or minus some amount of volts or milivolts of signal.
It would be highly desirable to reduce a difference between an absolute value of a power supply ground Vss located outside a chip and a power supply ground inside the chip wherein the chip is characterized by a high current, high frequency noise environment.